Carboxylic acids are useful feedstocks for the production of alkenyl carboxylates and alkyl carboxylates. Thus, for example, acetic acid is used to manufacture vinyl acetate or ethyl acetate. Acetic acid may be produced by the catalytic oxidation of ethylene and/or ethane.
Vinyl acetate is generally prepared commercially by contacting ethylene and acetic acid with molecular oxygen in the presence of a catalyst active for the production of vinyl acetate. Ethyl acetate is generally prepared by contacting ethylene and acetic acid in the presence of a catalyst active for the production of ethyl acetate.
Integrated processes for producing acetic acid and/or vinyl acetate are known in the art. EP-A-0 877 727 discloses an integrated process for the production of acetic acid and/or vinyl acetate from a gaseous feedstock comprising ethylene and/or ethane. The integrated process comprises a first step wherein ethylene and/or ethane is catalytically oxidised in a first reaction zone to produce a product stream comprising ethylene, unreacted ethane, acetic acid and water. This product stream may be passed directly to a second reaction zone and contacted therein with a molecular oxygen-containing gas in the presence of a suitable catalyst to produce vinyl acetate.
Research Disclosure 2244 of 1992 (June) No. 338 describes a process for the oxidation of ethane and/or ethylene to produce acetic acid in which the by-product carbon monoxide is oxidised to carbon dioxide. According to this document, the unreacted ethane and/or ethylene may be recycled to the oxidation reactor. Alternatively, the ethylene may be reacted with acetic acid to produce ethyl acetate or with acetic acid and oxygen to form vinyl acetate.
In the manufacture of vinyl acetate or ethyl acetate from ethylene and acetic acid, the molar ratio of the ethylene to acetic acid is desirably unity or approximately unity. Thus, in an integrated process in which ethane is oxidised to produce ethylene and acetic acid, the molar ratio of ethylene to acetic acid produced in the oxidation reaction is also desirably unity or approximately unity.
In the partial oxidation of a gaseous feedstock comprising ethylene and ethane, the product stream generally comprises ethylene, unreacted ethane, acetic acid and water. This product stream may be condensed and/or scrubbed to produce a gaseous stream comprising ethylene and ethane and a liquid stream comprising acetic acid and water. The gaseous stream comprising ethylene is typically recycled to the oxidation reactor. Continuous recycling of such an ethylene-containing stream will result in a build-up of ethylene in the oxidation reactor. As the concentration of ethylene increases the oxidation catalyst will become more selective towards acetic acid and the carbon oxides thereby deviating from the desired ethylene to acetic acid molar ratio of unity or approximately unity.